Turbine control system with early valve actuation under unbalanced conditions

ABSTRACT

A large steam turbine-generator control system with normal valve positioning systems for speed and load control and fast valve closure systems for turbine overspeed control, wherein the system also includes early valve actuation relay logic designed to function upon logical combinations of unbalance and time-rate-ofchange of unbalance of selected turbine-generator measured operating conditions such as mechanical input power, electrical load and generator terminal current.

United States Patent 1 1 3,60 1,6 1 7 [72] Inventors Francisco P. De Mello 2,803,785 8/1957 Desch 290/40 X Burnt Hills; 3,274,443 9/1966 Eggenberger et al.. N 290/40 X Markus A. Eggenberger, Schenectady; 3,421,014 1/1969 Moorganov 290/40 Richard Mlns' Sums Primary Examiner-Cris L. Radar 211 App]. No. 41,271

. Assistant ExammerW. E. Duncanson, Jr. [22] Filed May 28, 1970 1 A!!0rneys-W|lllam C. Crutcher, Bryan C. Ogden, Frank L.

{45] Patented Aug. 24,1971 N h 0 B w dd d1 hB F a [73] Assignec General Electric Company cu auser Scar a e an osep mm H [54] TURBINE CONTROL SYSTEM WITH EARLY VALVE ACTUATION UNDER UNBALANCED CONDITIONS 10 Claims 8 Drawin Fi s.

g 8 ABSTRACT: A large steam turbine-generator control system [52] US. Cl 290/40 C with normal valve positioning Systems f Speed and load [51 1 Cl F0 M 21/00 trol and fast valve closure systems for turbine overspeed con- [50] Field of Search 290]], 40, "0], wherein h system a|so includes early valve actuation 52 relay logic designed to function upon logical combinations of unbalance and time-rate-of-change of unbalance of selected [56] References cued turbine-generator measured operating conditions such as UNITED STATES PATENTS mechanical input power, electrical load and generator terl,792,241 2/1931 Ray l. 290/40 minal current.

I l MAIN STEAM LINE UNBALANCE LOAD 1 LOAD REF. FROM BOILER RELAY L0G": REFERENCE {I[| LOADING RATE 4 TURBINE CONTROL ELECTRIC (ADMISSIONlVALVES POWER CROSSOVER REHEAT MODIFIER PRESSURE LP L.P. LP. L.P. m

Tuna. run. ru run run. GENERATOR EMERGENCY 3 TRIP s- EL. AM CONDENSER REtlXI ER F 1 SPEED ERROR 9 2| SIGNAL 1 H REGULATION o o I COMPENSA'HON REG. NETWORK CONTROL VALVES EMERGENCY TRIP BIAS TRIGGER Anna INTERCEPT 2 35%)? RAM VALVE FLOW SIGNAL Q NON LINEAR FEEDBACK PATENTEU AUB24 IHYi MISMATCH (PRESSURE VERSUS CURRENT) OR RATE OF CHANGE OF MISMATCH GENERATOR POWER PER UNIT OF RATED REH EAT PRESSURE SHEET 3 OF 4 +.4'- -ACTUAT|ON LINE 'l'l Tca| 6 0:2 o'.4 ole o'.a 1150 H2 TIME t (SECONDS) l9 f n Q 45\ 41 PFER UNIT o RATED I X 6 l m l YES 5| LATCH CURRENT 2 I 5 2% T 27 I 22 K YES I I X 52 RESET J TO|7 I fi INVENTORS.

FRANCISCO P. DEMELLO. MARKUS A EGGENBERGERT RICHARD J. MILLS,

BY w -H 'THEIR ATTORNEY.

PATENTEUAUBZMBYI 3.60161/ T T FIG.8

. 6 v 64 70 67 VOLTAGE g I COMPARATOR I I ft 22 i 66 39. I 1 53 T T 65 I 64 v 68 I J VOLTAGE I x COMPARATOR I l I I RELAY 1 66 COILS TO l3,l7

T T INVENTORSI FRANCISCO P. DEMELLO, MARKUS A. EGGENBERGER,

. RICHARD J. MILLS,

BY Lu- 6 W64,

THEIR ATTORNEY.

TURBINE CONTROL SYSTEM WITH EARLY VALVE ACTUATION UNDER UNBALANCED CONDITIONS Background of the Invention This invention relates to turbine valve control systems for protection of the turbine and associated equipment under unbalanced conditions. More particularly, the invention relates to early valve actuation under the control of a logic system which provides overspeed protection and aid to system stability without producing undesirable valve actions during temporary power system transient swings.

Modern designs of large steam turbine-generators with low inertia-to-power ratios and more stringent reliability requirements have moved toward more sophisticated turbine control systems. Also there has been considerable interest in the utility industry concerning coordination of turbine controls with electrical power system requirements as an aid to electrical system stability.

In the past, turbine manufacturers have been primarily concerned with overspeed protection, and the controls have henceforth been designed to protect the turbine in the event of a load rejection." A load rejection is defined here as the sudden and permanent loss of all or part of the electrical load on the generator as occurs, for example, when opening the main generator circuit breaker. Following a load rejection, the electrical load on the generator (output) will be very low (essentially zero), while the mechanical turbine power (input), roughly proportional to steam pressure, will be substantially the same. until the valves reduce the steam flow. The resulting unbalance will accelerate the rotor and cause overspeed.

It has been suggested in US. Pat. No. 3,198,954 to M. A. Eggenberger et al. that a predetermined condition of un balance can anticipate and reduce potential rotor overspeed by initiating a reduction in the valve opening before overspeed takes place.

Another type of major system upset is a fault (short circuit) on some portion of the transmission system. In the short time period immediately following a fault, the load on the generator will drop, similar to the ease of a load rejection, but contrary to a load rejection situation, the generator terminal current will rise suddenly due to the fault. After the fault has been cleared, the attendant change in network conditions can cause oscillations in the terminal current (and electrical load) as the system attempts to return to normal operation. If the system is inherently stable, it is not necessarily desirable to actuate closing of the turbine valves during the fault or after the fault has been cleared.

It has been suggested that schemes using time delays can be employed to eliminate undesired tripping of the valves on short duration of mechanical/electrical power mismatches, but there schemes may act to deteriorate turbine overspeed protection. Other more elaborate schemes have been suggested in the prior art, using circuit breaker positions and/or distance relay interfaces, but these will create controls which varied in each system application, and are not considered practical for general application.

A turbine control system which is actuated by unbalance between mechanical power and electrical load is normally unable to distinguish between the case of a load rejection" and a fault, whereas different types of valve actions might be desired for these two situations, Another known system has suggested use of unbalance between mechanical power and current at the generator terminals to rapidly close the valves. This arrangement does distinguish between a load rejection and a fault, but may be apt to close the valves unnecessarily during swings of the electrical current following clearing to the fault when transiently unbalanced conditions exist.

Accordingly, one object of the present invention is to provide an improved turbine control system with provision for different types of early valve actuation to best suit the type of system unbalance which has occurred.

Another object of the invention is to provide an improved turbine valve control system which provides overspeed protection and capability to aid system stability, without being susceptible to undesired actuation during nonharmful transient conditions.

DRAWING The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of practice, together with further objects and advantages thereof, may best be understood by reference to the following description, taken in connection with the accompanying drawings, in which:

FIG. 1 is a simplified schematic view of a reheat steam turbine-generator and its load control system,

FIG. 2 is a functional logic diagram of one type of unbalance relay in the present invention,

FIG. 3 is a simplified representation of a power system under fault conditions,

FIG. 4 is a graph showing time variation of various operating variables of a conventional turbine-generator following the occurrence and clearing of a fault,

FIG. 5 is a graph showing the response of variables used in the relay logic of FIG. 2 as they behave during the fault and following its clearing,

FIG. 6 is a functional logic diagram of a modified type of relay, and

FIGS. 7 and 8 examples of suitable devices for use in the logic systems.

Summary of the Invention Briefly stated, the invention employs a control system responsive to logical combinations of unbalance and time rate of change of unbalance between various pairs of normally balanced variables to effect early turbine valve actuation for overspeed protection or as an aid to electrical system stability.

Description of the Preferred Embodiment FIG. 1 of the drawing shows a typical reheat turbine-generator and its associated control system suitable for use with the subject invention. Steam for the boiler passes through high pressure, reheat (intermediate pressure) and low pressure turbines l, 2 and 3 respectively. Means for controlling the steam flow include main stop valves 4, control (admission) valves 5, reheat stop valves 6, and intercept valves 7. The turbines drive generator 8 which supplied the transmission line via a circuit breaker 9.

The load control system for the turbine-generator is shown generally at 10, on the left. A controlling signal is introduced at lll representing the speed error, or difference between generator shaft frequency and speed reference. After modification of the speed error signal by a factor representing the setting for speed regulation in devices 12, a load reference signal from a source 13 representing a desired load on the turbine-generator is summed with the modified speed error signal and a resulting valve position signal representing desired valve position appears on lines 14. Electrohydraulic servo mechanisms, such as indicated generally at 15 for typical control valve (others indicated at 154) and at 16 for the intercept valves, place the valves in the proper position for the desired load. Additional details of the foregoing type of control system may be seen in US. Pat. No. 3,097,488 issued to M. A. Eggenberger et al. on July 16, 1963, incorporated herein by reference.

The valve positioning servo mechanisms 15, 16 include trigger and dump valve actuators 17, 18 respectively which, upon introduction of appropriate electrical signals, actuate solenoid-operated dump valves to release hydraulic oil from the bottom of the valve positioning rams, allowing the valves to close. See US. Pat. No. 3,495,50l issued Feb. 15, 1970 to Jens Kure-jensen for details of a suitable fast-closing valve actuator. Fast closure times for a typical valve are on the order of 0.1 seconds.

Control over the load reference source I3 and the dump valve triggers. I7, 18 comes about through actuation of the desired logic by the "unbalance relay logic device 19 which is the subject of the present invention. The unbalance relay logic 19 has provision for introducing input variables representative of turbine-generator-operating conditions. One such input is provided by a pressure transducer 20 measuring reheat steam pressure, which is proportional to mechanical input power to the turbine-generator shaft under steady state conditions. A wattmeter device 21 connected to the generator output terminals provides a signal proportional to electrical power output of the generator (kw. load), as well as a signal proportional to the generator terminal current.

As wili become apparent during the following description, one embodiment of the present invention employs only reheat pressure and generator current to actuate the unbalance relay, while a modification employs three input quantities, reheat pressure, generator current and electrical power (kw.).

Referring now to FIG. 2 of the drawing, a functional logic diagram is illustrated for the unbalance relay system I9. A first input signal 20 proportional to reheat steam pressure and the second input signal 21 proportional to generator terminal current are compared at 22. This pair of signals is normally matched under normally steady-state turbine-generatoroperating conditions. A signal representative of the difference or unbalance between the two input quantities appears at 23. This difference signal is designated herein as X.

The signal X on 23, representing a steam pressure and generator current mismatch (under unbalanced conditions), is fed to two logic channels. In the upper branch, the signal is dif ferentiated with respect to the time at 24 to provide a signal X representative of time rate of change of unbalance at 25.

The differentiation device 24 is preferably implemented by a discriminator network in a manner that passes instantaneous changes in signal 23 with a unity gain, while at the same time attenuating slower changes as indicated by the transfer function. This device allows discrimination between step changes (as in load rejections) and slower changes caused by system swings. This type of differentiator also prevents the undesirable amplification of high frequency noise. The differentiated quantity X is subjected to a comparator device 26, here preferably a voltage comparator, which provides an output in the event that the time rate of change of mismatch is greater than a selected quantity, here 0.4 per unit per second (or 40 percent per second). This selected quantity is adjustable so hat the threshold value may be set, at which an output will be obtained.

In tlr lower channel, the undifferentiated value X of the mismatch is also introduced to a similar comparator device 27, which provides an output in the event that the mismatch X is greater than a selected threshold quantity, here also selected as 0.4 per unit (or 40 percent.)

The two outputs from comparator devices 26, 27 are both inputs to a logical AND 28, which in turn provides an output signal to load reference source 13 and to control valve trigger 17 only in the event that both of the aforesaid logical conditions are met. The output signal acts to reduce the load reference and to close the control valves by setting the load reference signal to zero and actuating the control valve trigger l7.

The logical AND device 28 is provided with latch and reset inputs to keep device 28 actuated (after its initial actuation until the input to comparator device 27 falls below its threshold value, at which time logical AND device 28 is reset.

It will be understood to those skilled in the control art that the actual implementation of the unbalance relay system 19 shown in the logic diagram of FIG. 2 may take many forms in actual practice.

For example, the relay system 19 can be analog in nature, in which case the inputs are DC voltages summed at a summing junction, the differentiator 24 can be a passive RC network,

the comparator devices 26, 27 can be biased diodes, and the AND device can be comprised of simple electromechanical contacts in series.

It will also be understood to those skilled in the art that the functional diagram shown in FIG. 2 can be completely accomplished with special digital circuits or completely in terms of computer programming instructions for a general purpose digital computer. In the latter case, samplings are taken of reheat pressure and generator current, converted to digital quantities, and subjected to the various logical decision processes set forth in the diagram. The differentiation process represented at 24 is accomplished with a difference equation algorithm operating on successive samples of the periodically measured quantities. The comparator and AND functions 26, 27, 28 are accomplished by means of simple quantitative comparisons and conditional branch statements in the particular computer language used.

By way of specific example, although in no way intended to limit the scope of the present invention, FIGS. 7 and 8 show one preferred manner of implementing the logic diagram of FIG. 2.

FIG. 7 shows a discriminator circuit which may be employed as the differentiator 24 of FIG. 2. Unbalance signal X introduced at input terminal 60 will provide a rate of change of unbalance signal X at the output terminal 61. The transfer function of the circuit is as indicated inside box 24 in FIG. 2 with a time constant T,. Variation of time constant T is accomplished in the circuit of FIG. 7 by adjusting resistance 62.

FIG. 8 shows a logic circuit suitable for devices 26, 27 and 28 of FIG. 2 employing relay closures actuated by two voltage comparators 63. Signals X and X applied at input terminals 64 will provide output currents through relay actuating coils 65 when the inputs exceed threshold values set by means of variable resistors 66. As previously mentioned, each adjusta' ble threshold is preferably set at 0.4 per unit of rated values.

The arrangement of relay closures serves to implement the logic. Actuation of both output coils 65 provides closure of se ries-connected relay contacts 67, 68 to actuate output coil 69. The latch and reset is provided by relay closure 70 as should be apparent from the drawing.

Operation of FIG. 2 Embodiment For an understanding of the operation of the present inven tion, it is first necessary to consider the operation of a conventional turbine-generator under system disturbances. The problem of a load rejection has been previously explained wherein the power and terminal current both collapse together. The situation in the case of a fault on the transmission line is more complicated, and a better understanding is facilitated by reference to the simplified drawing of FIG. 3. There the generator 8 is shown connected to a simplified transmission system 30. Remote tie line breakers are indicated at 31 and 32. Representation of a fault or grounding of one line of the transmission system 30 at time t 0 is represented by a switch 33. Thereafter, the normal fault clearing relays cause opening of the breakers 31, 32 at a small but finite time interval T also indicated. The ordinate is provided with various scales quantifying the particular parameter identified at the initial balanced state at time t= 0. Mechanical power (or input power) represented by reheat pressure, is shown as line 35 and is relatively constant. The electrical load (or generator output power indicated as line 36 which drops rapidly until the time of fault clearing, and thereafter oscillates after the fault is cleared. Generator terminal current is indicated by line 37 which rises rapidly at first during a surge of current toward the fault and then decays due to generator demagnetization, until the fault is cleared. Thereafter, it drops in value and oscillates as indicated by curve.

Turbine-generator rotor speed is indicated byline 38. Due to the initial unbalance of mechanical input power and electrical output power caused by the fault, the rotor suffers an in crease in speed until clearing of the fault. Thereafter, the speed will oscillate as it seeks a steady value.

balance X under the situation of a fault such as that indicated in FIG. 4. The time I in seconds is the abscissa, while the mismatch (or rate of change of mismatch) between reheat pressure and the generator current in terms of per unit (or per unit per second) values is represented on the ordinate. The value of the mismatch X is shown by the solid line 39, while the rate of the change of mismatch X is shown by dotted line 40. As will be apparent from the logic of FiG. 2, these quantities must be greater than 0.4 per unit and 0.4 per unit per second respectively (below the actuation line 41) before the logical AND device 28 is actuated and there is a corresponding output from unbalance relay 19. As indicated in FIG. 5, the rate of change of mismatch signal X is below the actuation line 41 immediately following clearing of the fault. Subsequently, at a time approaching one second, the absolute value of mismatch of X, represented by line 39, is below the actuation line, but at this later time, the rate of change of mismatch of X is not severe enough to cause the unbalance relay to fulfill both logical conditions. Therefore, the unbalance relay 19 of the present invention is not actuated, whereas it might have become undesirably actuated during a transient back swing of the current with prior art devices if the rate factor X were not present in the disclosed logic system.

The action of the unbalance relay shown in FIG. 2 is such that the unbalancerelay is actuated during a load rejection when both the values of mismatch and the rate of change of mismatch are greater than specified threshold values. These specified threshold values may be altered individually for particular control conditions.

The action of the aforedescribed unbalance relay I9 is to trip the control valves by actuating trigger l7 and to also set the load reference to zero for both the control valves and intercept valves by altering the outputs from load reference device 13, so that both of the control valves and intercept values will close. This type of valve actuation is primarily to control overspeed. However, the unbalance relay logic 19 is not actuated either during a fault or during a transient backswing of current following a fault which might otherwise resemble a load rejection under previous systems.

Following a load rejection which actuates the unbalance relay 19, thus closing the turbine control and intercept valves, it would not be desirable to reopen the control valves until the mismatch X is reduced to less than the selected quantity (here 0.4 per unit or 40 percent). The mismatch X may be reduced either by restoring the electrical load or by reducing the reheat pressure. The purpose of the latch and reset inputs to the AND device 28 is to keep this device actuated (after its initial actuation) until the absolute mismatch X is reduced to less than the selected value, regardless of the rate of change signal X. Table I sets forth the conditions and type of action taken by the unbalance relay 19.

TABLE I-Continued Event X 0.4 X' 0.4 Action During Transient Backswing following fault Yes No None Following Load Rejection Yes Any None; control valves remain in tripped position and load ref. remains zero Following Load Rejection No Any Allow control valves to reopen; restore load ref. to control and intercept valves "AND" device 28 latched.

MODIFICATION Reference to FIG. 6 of the drawing illustrates a modification of the unbalance relay logic, designated 19, which can be substituted for relay logic 19 in FIG. 1. Inputs are signals at 20 representative of reheat pressure at 21 representative of generator current, and an additional input at 42 representative of generator electrical power. A first comparison is made of normally balanced conditions of generator electrical power .and reheat pressure at 43, and a signal representing the difference or mismatch therebetween, hereinafter designated as Y, appears at 44. This mismatch is also indicated as Y in FIG. 4. Signal Y passes to two channels as before, and is differentiated in the upper channel by a differentiator 45 to provide a time rate of change of mismatch Y. Both the mismatch Y and the time rate of change of mismatch Y are introduced to comparator devices 46, 47 respectively as before. Devices 46, 47 provide outputs to a logical AND device 48 if both of the values Y and Y exceed selectable threshold values, here shown as K, and K respectively. The values of K, and K are selected in a manner similar to the use of settings 66 in FIG. 8. The output from AND device 48 provides one input to an OR device 49.

Referring back to the input end of unbalance relay 19, reheat pressure and generator current are compared as before at 22, and the difference signal X is acted on by devices 24, 26 and 27 in a manner identical to the corresponding devices in relay logic 19. The selectable threshold values for devices 26 and 27 are indicated as K which can be chosen as 0.4 per unit in a typical system. The output of comparator devices 26 and 27 are introduced into an AND device 50. The output of AND device 50 is introduced into a latching relay 51 and also as a second input to the OR device 49. The relay 51 latches itself such that it will remain set until reset by the signal 52 from device 27. The purpose of the latching relay 51 is the same as for the corresponding AND device 28 of relay logic 19.

It will be seen that the modified unbalance relay logic provides basis for two entirely different types of valve actions according to the fulfillment of the conditions as indicated.

First, it will be observed that upon fulfillment of both the mismatch and the rate of change of mismatch of generator power and reheat pressure exceeding the threshold values K K respectively, there will be an output from AND 48 to intercept valve trigger 18 (see FIG. 1). This will provide a signal to close the intercept valves in the event of a fault and under selected circumstances will aid in system stability.

Under the more severe case of a load rejection, there is additionally an output from AND device 50 to latching relay 51 and to OR device 49, which results in signals to also actuate the control valve fast closure trigger 17, to alter the load reference source 13, and to actuate the intercept trigger 18 (see FIG. 1), thus controlling turbine overspeed.

Under a third condition of system disturbance, where it is not desired to actuate the unbalance relay 19, i.e., during temporary transient swings of current and load, it should be apparent that there will be no output from comparator device t .ll from comparator device 26. since the rates of change of mismatch (Y' and X respectively) are not sufficient to exceed the specified values. Therefore, there is no output at all from the unbalance relay, because the logical AND requirements 5. The combination according to claim 1, wherein said first and second control means each including a voltage comparator arranged to compare said mismatch signal with said threshold value and having additional means to adjust said transfer function Tsll+Ts with means for adjusting the time constant T.

are not met in either of the and devices 48, 50. threshold value to determine the degree of mismatch at which it should he further recognized that the ni iriiin rh t in said first or second signal is supplied from the first or second K, and K, required to actuate comparator devices 46 and 47 Control means p i yr iv l may b Set indcpgndently f h K Settings f 6. The combination according to claim 1 further including devices 26 and 27. Thus the severity of a transient fault latching "l Operated by Said first AND means to maintain required to actuate relay 19' may be set independently of th 10 said actuating signal. and also including reset means operated severity ofa load rejection required for actuation of the relay. y Said first Control means when Said first Value falls below Note also that the time constant T of differentiator 45 may be Said threshold Value and arranged to unlatch the latching set independently ofT in differentiator 24. The effect of these means and thercbyfermlnaie f actuating signal independent adjustments is to allow the setting of relay 19' to The Combinano" according clam 1 and further mclud' be tailored for the specific needs of each application. ing:

Reference to Table ll shows that one type of more drastic third control means Providing a third Signal respohswe to a early valve actuation is taken in the case of load rejection, sficond value of mlsmatch between? a secorld of with a less drastic (but nevertheless desirable) early valve acmany matchul Signals repreemmg turbmegeneramr' tion for severe temporary faults, and no action at all during Operating conditions when said second value exceeds a transient backswing following a fault. th'rd selected threshold value TABLE II During load rejection Yes"... Yes. Yes..." Ycs Trip control valves, intercept valves, set load reference to control and intercept valves; relay 51 latches.

During fault No No Yes Yes. Actuate fast closure of intercept valves only.

During fault clearing... N0 Yes Any No Allows LV. to reopen.

During transient baekswing to g fault Yes..." No. Yes... No None.

Following load rejection l Yes... Any' Anyml No Allows LV. to reopen. OX. remain closed. Load reference remains at zero.

D0 No Any... Any"... No Allow control valves to reopen; restore loacl reference to control and intercept valves.

Relay 51 latched.

Thus it will be seen that an early valve actuation control fourth control means providingaforth signal responsive to a system has been disclosed which will distinguish between the time rate of change of said second value of mismatch more severe case of load rejection and a fault, as well as one which exceeds a forth selected threshold value, and which will not be undesirably actuated upon transient swings second logical AND means providing an output to a second following a fault. Moreover, the modified system provides adone of said fast closing triggers when both of said third ditional flexibility in providing discrimination between load and forth signals are applied thereto rejection and fault conditions with a different type of valve ac- The Combination according I l im 7, wh r in Sai first tion in each circumstance. pair of turbine-generator-operating conditions are steam pres- While th ha b d ib d h i h i id d to 40 sure and generator terminal current and wherein said second he the preferred embodiment and one alternate form of the inpair of operating conditions are steam pressure and generator vention, other modifications will occur to those skilled in the real 10adart, and it is intended to cover in the appended claims all such The Combination according to claim 7, further including modifications as fall within the true spirit and scope of the in means connected to Provide an additional output from Said mi second logical AND means to said first fast closing trigger,

wh i l i d i whereby said first trigger is actuated by an output from either 1. In a turbine-generator control system having a plurality of the first or the Second logical AND means While Said Second steam valves with means to position said valves to control the trigger is actuated y an output from Said Second logical AND turbine, and further having fast closing triggers arranged to meahsreduce steam flow through selected valves rapidly in response a turbine'generator Control System having a plurality to actuaring signals h combination of steam turbine valves with means to position said valves to irst comm] means providing a fi signal responsive to a control the turbine and further having fast closing triggers arfirst value of mismatch between a first pair of normally ranged to reduce E flow through sejectred valves p y in matched signals representing turbine-gencrator-operating response to acwatlhg Slgna ]S1 the comblnarlorl Ofl conditions, when said first value exceeds a first Selected a first relay logic circuit responsive to a pair of normally threshold vafiley matched signals representative of steam. pressure and second control means providing a second signal responsive generiftor current aI 1d Provldmg a first P! Signal when to a time rate of change of Said first value of mismatch the mismatch and time rate of change of mismatch both which exceeds a second selected threshold value, and exceed Selected j l valuesr first |ogical AND means providing an actuating Signal to a a second relay logic circuit responsive to a pair of normally first one of said fast closing triggers when both of said first matched slgnals representatfvfi of steam Pressure and and Second g g are appfied to said AND "ream generator real load and providing a second output signal 2. The combination according to claim I, wherein said first when the mlsmatch and rate of change of pair of turbine-genorator-operating conditions are steam presboth exceed Selected threshold Values, sure and generator termin l current a first trigger logic circuit responsive to said first relay logic 3. The combination according to claim 1, wherein said first Circuit and arranged to effect one yp of turbine Valve pair of turbine-generator-operating conditions are steam presactuation to control overspeed, and sure and generator real load, a second trigger logic circuit responsive to either or both of 4. The combination according to claim 1, wherein said said first and second relay logic circuits and arranged to second control means includes a discriminator circuit having a another yp of turbine Valve actuation aid system stability. 

1. In a turbine-generator control system having a plurality of steam valves with means to position said valves to control the turbine, and further having fast closing triggers arranged to reduce steam flow through selected valves rapidly in response to actuating signals, the combination of: first control means providing a first signal responsive to a first value of mismatch between a first pair of normally matched signals representing turbine-generator-operating conditions, when said first valUe exceeds a first selected threshold value, second control means providing a second signal responsive to a time rate of change of said first value of mismatch which exceeds a second selected threshold value, and first logical AND means providing an actuating signal to a first one of said fast closing triggers when both of said first and second signals are applied to said AND means.
 2. The combination according to claim 1, wherein said first pair of turbine-generator-operating conditions are steam pressure and generator terminal current.
 3. The combination according to claim 1, wherein said first pair of turbine-generator-operating conditions are steam pressure and generator real load.
 4. The combination according to claim 1, wherein said second control means includes a discriminator circuit having a transfer function Ts/1+Ts with means for adjusting the time constant T.
 5. The combination according to claim 1, wherein said first and second control means each including a voltage comparator arranged to compare said mismatch signal with said threshold value and having additional means to adjust said threshold value to determine the degree of mismatch at which said first or second signal is supplied from the first or second control means respectively.
 6. The combination according to claim 1 further including latching means operated by said first AND means to maintain said actuating signal, and also including reset means operated by said first control means when said first value falls below said threshold value and arranged to unlatch the latching means and thereby terminate the actuating signal.
 7. The combination according to claim 1 and further including: third control means providing a third signal responsive to a second value of mismatch between a second pair of normally matched signals representing turbine-generator-operating conditions when said second value exceeds a third selected threshold value, fourth control means providing a forth signal responsive to a time rate of change of said second value of mismatch which exceeds a forth selected threshold value, and second logical AND means providing an output to a second one of said fast closing triggers when both of said third and forth signals are applied thereto.
 8. The combination according to claim 7, wherein said first pair of turbine-generator-operating conditions are steam pressure and generator terminal current and wherein said second pair of operating conditions are steam pressure and generator real load.
 9. The combination according to claim 7, further including means connected to provide an additional output from said second logical AND means to said first fast closing trigger, whereby said first trigger is actuated by an output from either the first or the second logical AND means while said second trigger is actuated by an output from said second logical AND means.
 10. In a turbine-generator control system having a plurality of steam turbine valves with means to position said valves to control the turbine and further having fast closing triggers arranged to reduce steam flow through selected valves rapidly in response to actuating signals, the combination of: a first relay logic circuit responsive to a pair of normally matched signals representative of steam pressure and generator current and providing a first output signal when the mismatch and time rate of change of mismatch both exceed selected threshold values, a second relay logic circuit responsive to a pair of normally matched signals representative of steam pressure and generator real load and providing a second output signal when the mismatch and time rate of change of mismatch both exceed selected threshold values, a first trigger logic circuit responsive to said first relay logic circuit and arranged to effect one type of turbine valve actuation to control overspeed, and a second trigger logic circuit responsive to either or both of said first and second relay logic circuits and arranged to effect another type of turbine valve actuation to aid system stability. 